qgis
I made a grid layer over a country and the data display on it is ID, xmax, xmin, ymax and ymin. It's clear that the last 4 parameters are the dimensions of the square of each ID, but what is the reference? I was expecting a latitude/longitude coord and I dont know how to work with these ones.
Open Python console and show the editor, and paste the following code.
I suppose that segALD00 is the name of a field which you want to update with a derivative of its own value, and that longkm is another field.
The script defines a local function upd_value, which does the calculation of the new value.
# definition of the function which calculates new values
def upd_value(feat, value):
# calculations come here, e.g.
geom = feat.geometry().boundingBox()
result = value * (((geom.xMinimum() - geom.xMaximum())**2 +
(geom.yMinimum() - geom.yMaximum())**2)**.5)*102.47
return result
# get the active layer
layer = iface.activeLayer()
prov = layer.dataProvider()
# get the fields
fnm = prov.fieldNameMap()
# get an iterator for the features
feats = prov.getFeatures()
# calculate new values and update fields with fieldnames beginning
# with 'seg' in one single step
result = prov.changeAttributeValues({feat.id(): \
{fnm[fn]: upd_value(feat, feat[fn]*feat['longkm']) \
for fn in fnm if fn.startswith('seg')} \
for feat in feats})
The error report tell you that that you have entered illegal parameters. 
The values cant be over +/-180 for X and +/-90 for Y In the tool box, try setting the xmin and ymax to -180 and 90 respectively.
Since the GRASS tool takes these attributes from the layers, it might hint that there might be an underlying error with the source layer's CRS or extents.
Best Answer
The units shown are the project's currently selected coordinate system units.
Go to Project -> Properties -> General and investigate there. Specifically toggle "Coordinate display" to change the Coordinate display at the bottom of the map window to show coordinates in "Map units".